1. Field Of Invention
This invention relates to apparatus used in detecting and measuring the symmetrical components in multiphase AC electrical power systems and particularly to sequence filters which generate an output signal having a magnitude which is a function of the degree of imbalance in the power system.
2. Prior Art
It is common to represent the sinusoidal currents and voltages of a multiphase AC electrical power system as sets of rotating vectors, one set for the currents and another for the voltages. For a balanced system, the vectors of each set are equiangularly spaced and are of equal magnitude. For instance, in the commonly used three phase system the vectors of each set would be 120.degree. apart. Since it is desirble from an efficiency standpoint to maintain the system in balance, an effort is made to balance the loads applied to each phase of the system. Under these circumstances, a serious imbalance in the system is then an indication of an abnormal condition, such as a short circuit, a fault, a failure in or loss of a load, or a failure in the multiphase electrical source. Detection of the imbalance then can be used as a warning of an abnormal condition and/or a trigger for initiating protective or corrective action.
The operation of a class of multiphase AC power system imbalance detectors, known as negative sequence filters, is based upon the principle that any set of rotating vectors, whether balanced or unbalanced, can be resolved into sets of balanced vectors equal in number to the number of phases in the system. For the three phase system, the three balanced sets of vectors are known as the positive sequence set, the negative sequence set and the zero sequence set. The positive sequence set includes three rotating vectors of equal magnitude and spaced 120.degree. apart in phase which rotate in the same order as the line vectors. Thus in the case of a three phase system in which the phase order is a, b, c, the positive sequence vectors also rotate in the order a, b, c. The negative sequence vectors while also being of equal magnitude (although generally not of the same magnitude as the positive sequence vectors) and spaced 120.degree. apart in phase, rotate in the order a, c, b or in the opposite order of the line vectors. The vectors of the zero sequence set are equal in magnitude and are all in phase. For a balanced system, the magnitudes of the negative sequence vectors and the zero sequence vectors are zero and the positive sequence vectors are identical to the line vectors. However, when there is an imbalance in the system, the magnitude of the negative sequence vectors increases as a function of the degree of imbalance. For a more complete discussion of positive, negative and zero sequence vectors, see Circuit Analysis Of A-C Power Systems, Volume I, by Edith Clarke, John Wiley & Son, Inc., New York 1943, and Symmetrical Components, by L. J. Myatt, Pergamon Press, 1968, Library of Congress Catalog Card No. 68-26942.
The negative sequence filter extracts the negative sequence voltage or current from the power system and uses it to operate a threshold device such as a current relay when the associated system imbalance reaches a preset level. Reactive components are used in the filters to achieve the phase shifting required to generate the negative sequence components. Many prior art negative sequence filters use two reactive elements. An example of such a prior art negative sequence filter is disclosed in an article coauthored by the present inventor and P. F. Boggess entitled "Negative-Sequence Voltage Detection On Aircraft" and appearing as Transaction Paper No. 56-440 in the July, 1956 issue No. 25 of Applications And Industry published by the American Institute of Electrical Engineers. A functionally similar negative sequence filter is disclosed in U.S. Pat. No. 2,836,771. Another prior art circuit for generating the negative sequence voltage is disclosed at page 82 in Symmetrical Components cited above. While this circuit, which is used only for measuring the negative sequence voltage, employs only one reactive element for phase shifting, it requires a pair of transformers which, of course, significantly adds to the cost of the system. A notable shortcoming of these and other prior art negative sequence filters is that they are dissipative, which becomes important on systems of limited capacity such as aircraft electrical systems.
It is therefore a primary object of the present invention to provide a sequence filter which is simple in design, economical to manufacture and efficient in use.
It is also an object of the invention to provide a sequence filter which is easily tuned through the use of economical precision components.
It is a more specific object of the invention to provide such a sequence filter which only employs one reactive element.